Know you not, that to whom you yield yourselves servants to obey, his servants you are to whom you obey; whether of sin to death…
It was the custom to impress a distinctive mark or brand on the slaves belonging to different masters. A slave might thus, by no uncommon metonymy, be spoken of as belonging to a certain mark, the mark being put for the master whose mark it was; and when a slave was transferred from one master to another, as being delivered over to a new mark or brand, that is, to a new proprietor or master, to whom, or, by the same figure, to whose mark he was then to consider his person attached and his service and obedience due. This is probably the true meaning, "Ye have obeyed from the heart that mark [or brand] of doctrine to which ye have been delivered over"; this translation giving every word its full and proper effect. They passed from one service to another, distinguished by a new mark, to which, as reminding them of their new master, and the appropriate symbol of his property in them and his power over them, they were thenceforward to render their obedient service. The "doctrine" of Christ is the distinguishing badge, or appropriate mark, of all His servants. They bear the profession and impress of His truth; and, under the influence of that truth, they serve Him as the Master who has stamped its impression upon them, in a spirit of reverential love.
(R. Wardlaw, D. D.)
Parallel VersesKJV: Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?